Preheat oven to 375 degrees F (190 degrees C).
In a large bowl, gently toss together blackberries, white sugar, and lemon juice. Pour into a 9-inch pie plate.
In a separate bowl, combine flour, oats, brown sugar, cocoa powder, and salt. Cut in cold butter until mixture resembles coarse crumbs. Sprinkle over blackberries.
Bake in preheated oven for 30 to 35 minutes, or until topping is crisp and golden and the blackberries are bubbling. Serve warm.